For the connection of such wheeled agricultural implements, inter alia cutters, exact cutters and the like, to a tractor, coupling devices are required which, on one hand, ensure a stable towing connection and, on the other hand, provide a transfer of motive power from the power take-off of the tractor to the driven working means of the implement in such a manner that the losses of transfer become as small as possible and the maneuvering ability in the field as optimal as possible so that the implement may follow the ground and the crop, thereby running through the field as efficiently and quickly as possible.
In most of the prior coupling systems either the centrally mounted towing hitch of the tractor or a towing bar between the lifting linkage is used for directly coupling the drawbar of the implement through which the towing force is transferred to the implement. The driving power is simultaneously transferred from the power take-off of the tractor by means of a primary, releasable power transfer shaft to a secondary power transfer shaft journalled in connection with the drawbar and connected with the working means of the implement through a universal joint. The primary, releasable power transfer shaft provided with two universal joints and an intermediate telescopic part is a critical point of the coupling-up. To obtain the optimum degree of efficiency and correct geometry of rotation of the shaft the coupling point between the towing device of the tractor and the drawbar must be located approximately halfway between the power take-off of the tractor and the secondary power intake of the power transfer shaft which generally includes a connecting shaft. Since a large angle of swing is at the same time desired when turning in the field, the primary power transfer shaft has to be comparatively long which gives rise to vibrations and thus to increasing wear and tear and even damage. If the coupling point is moved from the center of the power transfer shaft, the power transfer shaft may, for the same angle of swing, be shorter. In return, however, the maximum angular motion within the universal joints increases, whereby also wear and tear and the risk of damage increases. This problem may partly be eliminated by use of a so-called wide-angle universal joint, but this solution is still insufficient if it is desired to manoeuvre at large angles of swing.
With a view to improve the maneuvering ability and to simultaneously reduce wear and tear in the power transfer a coupling device is known from Danish patent application No. 4283/80, according to which the towing force as well as the motive power is transferred from the tractor to the implement through a supporting unit for a mechanical transmission consisting of at least two parts mutually pivotal about a vertically extending axis. The one part of said supporting unit operates as a releasable pull connection with the tractor lifting linkage and as a journal for the input shaft of the transmission, respectively, said shaft being releasably connected to the power take-off of the tractor by means of a propeller shaft. The other part of the supporting unit operates as a permanent pull connection with the drawbar of the implement and as a direct or indirect journal bearing for the output shaft of the transmission, respectively, firmly connected with a mechanical transmission journalled within or along the drawbar and being in the form of a driving shaft or a belt drive transferring the motive power to the working means of the implement.
The prior coupling device allows for working at large angles of swing between the tractor and the drawbar of the implement without the risk of overloading the primary power transfer shaft. On the other hand, the solution is more complicated and somewhat more expensive than the conventional solution. Moreover, the flexibility in the feed-back to the tractor is obtained to the detriment of essentially reduced margin with a view to designing the drawbar and the implement proper. In view of the fact that the motive power is transferred further to the implement through a mechanical transmission suspended within or along the drawbar, and as the motive power is transferred therefrom to the implement proper through a drive shaft journalled concentrically with the substantially vertical pivot axis for the connection of the drawbar with the implement proper, the power intake constituted by said shaft is vertically positioned and firmly connected with the drawbar as well as with the main frame of the implement proper. The circumstance makes it particularly difficult to obtain a simple, reliable and economical structure of the implement with the working means carried out as a separate and relatively light unit that is suspended and can float within the main frame. This is frequently desired with a view to ensure that the working means effectively follow irregularities in the ground and at the same time are damaged as little as possible by collision with solid hindrances such as large stones or the like.
In order to obtain the same flexibility of the coupling-up between the tractor and the implement as of the coupling device according to the above mentioned patent application it is known as such to make use of a hydraulic transfer of power, a hydraulic pump being directly mounted on the mechanical power take-off of the tractor wherefrom a flexible hose connection is provided along the drawbar of the implement to a hydraulic motor coupled to the working means of the implement. This provides further for obtaining considerably more independence with respect to the design of the draw bar and the implement proper.
The connecting joint between the drawbar and the implement proper may thus be placed in the position most advantageous with a view to maneuvering, straight in front of the field of operation of the working means, without requiring a further transmission connection--as is the case with implements having mechanical transmission. Without causing difficulties with the transmission, the implement may further be provided with separate, floatingly suspended working means possessing the above specified advantages.
In the known coupling device with hydraulic transmission the flexibility of the connection between the tractor and the implement is further provided in that the drawbar of the implement is connected with the central towing hitch of the tractor across a particular pull extension so that the drawbar is pivotally connected with the pull extension secured to the tractor at a point located behind the rearmost vertical tangential plane of the tractor.
This solution makes, however, the connection and disconnection of the implement more difficult in that the pull extension must be mounted and dismounted separately if the tractor shall now and then be used together with other towed implements. Moreover, the yoke pull may vary somewhat from one tractor make to another which might necessitate an individual adaptation to the actual tractor type. The coupling and uncoupling is, however, made particularly difficult because the hydraulic pump that is comparatively heavy and connected with strong hoses shall be lifted between a parking fixture on the drawbar and the power take-off shaft of the tractor which during operation both supports and drives the pump. Consequently, the coupling and the uncoupling also includes mounting and dismounting of special fixtures preventing the pump from rotation or being released during working.
The most essential drawback of the prior coupling device with hydraulic transmission is, however, that it cannot be used in connection with agricultural implements having a high power requirement and which have become continuously more important, since it is not possible to mount the large and heavy pumps necessary for that purpose directly on the power take-off shaft of the tractor without involving unacceptably hard wear and tear and the risk of damage during operation.